Slidable bone plate system

ABSTRACT

The invention is directed to a bone plate system. The bone plate system comprises a base plate having two generally parallel elongated screw slots extending therethrough. Two bone screws are provided that are capable of securing the base plate to a bone by insertion through the screw slots into the bone. Each bone screw has a screw head and a threaded portion extending therefrom. An interference device is attached to the base plate and retains the bone screws while permitting the bone screws to toggle and to controllably slide in the screw slots of the base plate. This design is particularly useful for joining adjacent vertebral bodies, as it permits controlled settling of the vertebral bodies, thereby enhancing the healing process.

BACKGROUND

The spinal column of vertebrates provides support to bear weight andprotection to the delicate spinal cord and spinal nerves. The spinalcolumn comprises a series of vertebrae stacked on top of each other.There are typically seven cervical (neck), twelve thoracic (chest), andfive lumbar (low back) segments. Each vertebra has a cylindrical-shapedvertebral body in the anterior portion of the spine with an arch of boneto the posterior that covers the neural structures. Between eachvertebral body is an intervertebral disk, a cartilaginous cushion tohelp absorb impact and dampen compressive forces on the spine. To theposterior, the laminar arch covers the neural structures of the spinalcord and nerves for protection. At the junction of the arch and anteriorvertebral body are articulations to allow movement of the spine.

Various types of problems can affect the structure and function of thespinal column. These can be based on degenerative conditions of theintervertebral disk or the articulating joints, traumatic disruption ofthe disk, bone or ligaments supporting the spine, tumor or infection. Inaddition congenital or acquired deformities can cause abnormalangulation or slippage of the spine. Anterior slippage of one vertebralbody on another (spondylolisthesis) can cause compression of the spinalcord or nerves. Patients who suffer from one of more of these conditionsoften experience extreme and debilitating pain, and can sustainpermanent neurologic damage if the conditions are not treatedappropriately.

One technique of treating these disorders is known as surgicalarthrodesis of the spine. This can be accomplished by removing theintervertebral disk and replacing it with bone and immobilizing thespine to allow the eventual fusion or growth of the bone across the diskspace to connect the adjoining vertebral bodies together. Thestabilization of the vertebra to allow fusion is often assisted by asurgically implanted device to hold the vertebral bodies in properalignment and allow the bone to heal, much like placing a cast on afractured bone. Such techniques have been effectively used to treat theabove described conditions and in most cases are effective at reducingthe patient's pain and preventing neurologic loss of function. However,there are disadvantages to the present stabilization devices.

The spinal fixation device needs to allow partial sharing of the weightof the vertebral bodies across the bone graft site. Bone will not healif it is stress shielded from all weight bearing. The fixation deviceneeds to allow for this weight sharing along with the micromotion thathappens during weight sharing until the fusion is complete, often for aperiod of three to six months or longer, without breakage. The devicemust be strong enough to resist collapsing forces or abnormal angulationduring the healing of the bone. Loss of alignment during the healingphase can cause a poor outcome for the patient. The device must besecure in its attachment to the spine to prevent migration of theimplant or backout of the screws from the bone which could result indamage to the structures surrounding the spine, resulting in severe andpotentially life threatening complications. The device must be safelyand consistently implanted without damage to the patient.

SUMMARY OF THE INVENTION

The present invention is directed to a bone plate system that addressesone or more of the above drawbacks. In one embodiment, the bone platesystem comprises a base plate having two generally parallel elongatedscrew slots extending therethrough. Two bone screws are provided thatare capable of securing the base plate to a bone by insertion throughthe screw slots into the bone. Each bone screw has a screw head and athreaded portion extending therefrom. An interference device is attachedto the base plate and retains the bone screws while permitting the bonescrews to toggle and to controllably slide in the screw slots of thebase plate. This design is particularly useful for joining adjacentvertebral bodies, as it permits controlled settling of the vertebralbodies, thereby enhancing the healing process.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of a bone plate system according to theinvention.

FIG. 2 is a top view of the bone plate system of FIG. 1.

FIG. 3 is a side view of the bone plate system of FIG. 1.

FIG. 4 is an end view of the bone plate system of FIG. 1.

FIG. 5 is an end cross-sectional view of the bone screws insertedthrough the bone plate and covered by the interference device of a boneplate system as depicted in FIGS. 1 to 4.

DETAILED DESCRIPTION

An exemplary embodiment of a bone plate system according to the presentinvention is shown in FIGS. 1 to 5. The depicted bone plate system isparticularly useful for stabilizing two or more vertebral bodies toallow fusion by holding the vertebral bodies in proper alignment, andthus allowing the bone to heal. The bone plate system of the inventionis preferably weight-sharing over a period of at least about threemonths, and more preferably over a period of at least about six months.As used herein, the term “weight-sharing” as describing the system meansthat the system allows for partial sharing of the weight of one or morevertebral bodies across a bone graft site.

Generally, the bone plate system comprises a base plate 14, bone screws16, and one or more interference devices 18. The base plate 14 is aplate used to connect two or more bones. As used herein, the term“bones” is intended to include both bones and bone fragments orportions. The base plate 14 can be of any suitable shape or size for thedesired application.

In the depicted embodiment, the base plate 14 is a generally rectangularplate having two relatively long, generally parallel sides 20, a top end22 and a bottom end 24, with the top end and bottom end also beinggenerally parallel and shorter than the sides, as shown in FIG. 2. Inthe illustrated embodiment, each of the sides 20, top end 22 and bottomend 24 includes a region that is scalloped or concave. The concaveregions are preferably located along the midsections of each of thesides 20, top end 22 and bottom end 24. As discussed further below, thisscalloped or concave design is particularly desirable to prevent thebase plate 14, when implanted, from interfering with the esophagus ofthe patient.

When viewed from a side 20, the base plate 14 has a bottom surface thatis slightly concave (along the length of the base plate), as best shownin FIG. 3. Similarly, when viewed from the top end 22 or the bottom end24, the bottom surface of the base plate 14 is slightly concave (alongthe width of the base plate), as best shown in FIG. 4. This designpermits the base plate 14 to be curved in the sagittal and horizontalplanes to provide a better fit when implanted in a patient.

In one embodiment, the sides 20 each having a length ranging from about20 mm to about 40 mm, and the top and bottom ends 22 and 24 each havinga length ranging from about 14 mm to about 20 mm. The dimensions of thebase plate 14 can vary depending upon the size of the patient in whomthe plate is to be introduced. Further, the base plate 14 can have anyother suitable shape, such as an oval shape.

The base plate 14 can be made of any suitable material, and ispreferably made of titanium or a titanium alloy. If desired, the baseplate 14 can also be part of a larger device.

The base plate 14 contains at least two bone screw slots 30 forreceiving the bone screws 16. In the depicted embodiment, the base plate14 contains four bone screw slots 30 arranged in two pairs. The bonescrew slots 30 a of the first pair are located nearer the top end 22 ofthe base plate 14, and the bone screw slots 30 b of the second pair arelocated nearer the plate's bottom end 24. The bone screw slots 30 a ofthe first pair are generally parallel to each other and to the sides 20of the base plate 14. Similarly, the bone screw slots 30 b of the secondpair are generally parallel to each other and to the sides 20 of thebase plate 14. The first pair of bone screw slots are 30 a preferablygenerally aligned with the second pair of bone screw slots 30 b and thepairs of slots positioned relative to the sides 20 and top and bottomends 22 and 24 of the base plate 14 so as to create a generallysymmetrical arrangement about the center of the base plate, as bestshown in FIG. 2. The distance D1 between the pairs preferably rangesfrom about 6 mm to about 14 mm. The distance D2 between the slots 30 aof the first pair preferably ranges from about 4 mm to about 8 mm.

In the depicted embodiment, all of the slots 30 have the samedimensions. The length of each slot preferably ranges from about 4 mm toabout 8 mm, more preferably from about 6 mm to about 7 mm, still morepreferably about 6.6 mm. The width of each slot preferably ranges fromabout 3 mm to about 6 mm, more preferably from about 4 mm to about 5.5mm, still more preferably about 4.8 mm.

The depicted base plate 14 is particularly designed for the portion ofthe plate nearer the top end 22 to be attached to one vertebral body andthe portion of the plate nearer the bottom end 24 to be attached toanother vertebral body so that the plate is mounted in a verticalarrangement. The plate 14 has a bottom surface 26 that, in use, isadjacent to one or more of the bone surfaces, and a top surface 28 thatfaces away from the bone surface(s).

In the depicted embodiment, the bottom and top surfaces 26 and 28 aregenerally parallel to one another over a center region of the base plate14, which encompasses the slots 30, so that in the center region thebase plate has a generally uniform thickness, although the plate neednot have a uniform thickness in this region. The top and bottom ends 22and 24 of the base plate 14 are generally tapered, being thicker closerto the slots 30 and thinner closer to the edge of the plate 14, so thatthe bottom and top surfaces 26 and 28 are not parallel to one another atthe top and bottom ends. Preferably the center region of the base plate14, which encompasses the slots 30, has a thickness ranging from about 1mm to about 4 mm, more preferably from about 2 mm to about 3 mm, stillmore preferably about 2.8 mm. The tapered top and bottom ends 22 and 24each preferably taper to an edge thickness ranging from about 0.5 mm toabout 2 mm, more preferably from about 0.5 mm to about 1.5 mm, stillmore preferably about 1 mm. The top and bottom ends 22 and 24 preferablytaper at an angle ranging from about 20° to about 40°. This tapered enddesign is particularly beneficial in vertebral applications where thebase plate 14 comes into contact with the esophagus. With the inventivetapered design, the esophagus is not positioned over a sharp corner, asin many plate designs, but instead is provided with a smooth transitionsurface as it passes over the inventive base plate, minimizing damage tothe esophagus. In an alternative embodiment, only the top end 22 isprovided with the tapered design, as the top end has the leading edgeover which the esophagus interfaces. The tapered design in combinationwith the scalloped or concave design, discussed above, further enhancesthe ability of the plate system to avoid causing dysphagia.

The bone plate system of the invention includes a number of bones screws16 that corresponds to the number of bone screw slots 30, which in thedepicted embodiment is four. In use, the bone screws 16 extend throughthe bone screw slots 30 in the base plate 14 and are screwed into thebone. The bone screws 16 can be made of any suitable material, and arepreferably made of the same material as the base plate, such as titaniumor a titanium alloy. The shape and function of the bone screws aredescribed in more detail below.

One or more interference devices 18 are provided to retain the bonescrews 16 that extend through the bone screw slots 30 in the base plate14. By “retain” is meant that a interference device 18 covers asufficient portion of a bone screw 16 to prevent that bone screw from“backing out” out of the bone and base plate 14. The terminology“retain” is not intended to require that the entire top surface, or eventhe majority of the top surface, of a bone screw 16 be covered.

In the depicted embodiment, the bone plate system includes twointerference devices 18, each one for retaining a corresponding one ofthe two pairs of bone screws 16, so that one interference device 18 a isprovided nearer the bottom end 24 of the base plate 14 and the otherinterference device 18 b is provided nearer the top end 22 of the baseplate, as best shown in FIG. 1. Each interference device 18 is generallyrectangular having a rounded top edge.

The thickness of each interference device 18 preferably ranges fromabout 0.5 mm to about 2 mm, and more preferably from about 1 mm to about1.5 mm. As shown best in FIGS. 1 and 3, the edge of each interferencedevice 18 that is positioned nearest the corresponding top or bottom end22 or 24 of the base plate 14 is tapered in a manner similar to the topand bottom ends of the base plate.

In use, the interference devices 18 are removably fixedly attached tothe base plate 14. As used herein, the term “fixedly attached” refers tothe arrangement where the interference device 18, while being removablyattached to the base plate 14, cannot slide relative to the base plate.The base plate 14 is preferably provided with recesses 34 to receive theinterference devices 18 and retain them in the desired position. Theinterference devices 18 can be removably attached to the base plate 14by any suitable mechanism. In the depicted embodiment, each interferencedevice 18 is provided with a cover screw hole 36.

The base plate 14 is provided with two cover screw apertures 38 thatgenerally correspond is size and location to the cover screw holes 36 inthe interference devices 18 when the interference devices are correctlypositioned over the base plate. In the depicted embodiment, the coverscrew apertures 38 are offset from the bone screw slots 30 in the baseplate 14, as best shown in FIG. 1. By “offset” is meant that the coverscrew aperture 38 is provided at a position that does not fall on a linethat passes through the centers of the bone screw slots 30, but insteadthat is offset from such a line, preferably in a direction closer to thecenter of the base plate 14.

To removably attach an interference device 18 to a base plate 14, acover screw 39 is screwed into the cover screw hole 36 in theinterference device and a corresponding cover screw aperture 38 in thebase plate, as is generally known in the art. However, other mechanismsfor attaching a interference device to a base plate known to thoseskilled in the art could also be used in accordance with the invention.If a different mechanism for attaching an interference device 18 isused, the point of attachment to the base plate 14 is preferably offsetfrom the bone screw slots 30.

FIG. 5 shows how the bone screws 16 are positioned relative to a baseplate 14 and interference device 18. Each bone screw 16 comprises ascrew head 15 and a threaded portion 17 extending from the screw head.Each bone screw head 15 has a generally rounded top surface 40,optionally with a generally flat center region 42, and a generallyrounded bottom surface 44. The center region 42 of the generally roundedtop surface 40 includes a hexagonal or other shaped aperture 46, asshown in FIGS. 1 and 2, for receiving the end of an insertion tool, asis generally known in the art.

Within the base plate 14, each bone screw slot 30 has a generallyrounded edge surface 48 that interfaces with the bottom surface 44 of acorresponding bone screw head 15. An elongated axial opening 50 isprovided along the bottom of each bone screw slot 30 to receive thethreaded portion 17 of the bone screw 16 as the bone screw slides in theslot. The rounded edge surface 48 of the bone screw slot 30 joins thetop surface 28 of the bone plate 14 to a top edge of the axial opening50. The axial opening 50 has a width less than the width of the topportion of the bone screw slot 30, i.e., the portion of the bone screwslot receiving the screw head 15.

The interference device 18 has two generally rounded edge surfaces 52that interfaces with the edges of the generally rounded top surfaces 40of the bone screw heads 15 while not substantially covering the centerregion 42 of the bone screw head, to thereby prevent the bone screw 16from backing out of the base plate 14. This design permits each bonescrew 16 to toggle within its corresponding bone screw slot 30 while theinterference device 18 still exerts sufficient force on the bone screwto control the degree to which the bone screw slides within the bonescrew slot 30.

In the depicted embodiment, the rounded top surface 40 of each bonescrew 16 has a maximum outer diameter less than the maximum outerdiameter of the screw's rounded bottom surface 44. With this design, therounded bottom surface 44 can be made sufficiently large to prevent thehead of the bone screw 16 from passing through bone screw slot 30, whilethe rounded top surface 40 is smaller to permit the interference device18 to still lower on the base plate 14, thus further reducinginterference between the plate system and the esophagus.

In one embodiment, the interface of the top surface 40 of the bone screwhead 15 and the edge surface 52 of the interference device 18 isprovided to require a minimum force to cause the bone screw 16 to slidewithin the bone screw slot 30, thus permitting the bone screws tocontrollably slide in the bone screw slots. The minimum force preferablyranges from about 15 to about 50 pounds, more preferably from about 30to about 40 pounds. The system can be advantageously arranged to permitthe bone screw 16 to both settle in a direct vertical plane of 1.5 to 2mm and pivot under the interference device 18.

The offset location of the anchor or attachment point (i.e., the coverscrew) of the interference device 18 to the base plate 14 advantageouslyprovides increased resistance to screw sliding and toggling as vertebralbodies joined by the plate begin to settle. Specifically, when the bonescrews 16 are first introduced into vertebral bodies through the baseplate 14, they are introduced into the bone screw slots 30 closer to thetop and bottom ends of the base plate (i.e., farther from the center ofthe base plate). In this location, the bone screws are permitted totoggle and slide fairly easily, which is desirable because itadvantageous to permit the plate system to initially engage thevertebral bodies and permit the vertebral bodies to settle on the graftfairly quickly. After initial settling has occurred, the base plate 14moves relative to the bone screws 16, thereby positioning the bonescrews closer to the center of the base plate. As the bone screws movetoward the center of the base plate 14, they also move nearer the coverscrew or other point of attachment between the interference device andthe base plate. At this point of attachment, the interference deviceexerts the greatest force on the bone screws 16. Thus, as the bonescrews 16 move toward the point of attachment, the incremental forcerequired to further move the bone screws toward the point of attachment(and thus toward the center of the plate) increases. Thus, the platesystem advantageously controls further settling, thereby preventing thevertebral bodies from collapsing too quickly.

The preceding description has been presented with reference to presentlypreferred embodiments of the invention. Workers skilled in the art andtechnology to which this invention pertains will appreciate thatalterations and changes in the described structures may be practicedwithout meaningfully departing from the principal, spirit and scope ofthis invention. Accordingly, the foregoing description should not beread as pertaining only to the precise structures described andillustrated in the accompanying drawings, but rather should be readconsistent with and as support to the following claims which are to havetheir fullest and fair scope.

1. A bone plate system comprising: a base plate having two generallyparallel elongated screw slots extending therethrough; two bone screwscapable of securing the base plate to a bone by insertion through thescrew slots into the bone, each bone screw having a screw head and athreaded portion extending therefrom; an interference device attached tothe base plate and retaining the bone screws while permitting the bonescrews to toggle and to controllably slide in the screw slots of thebase plate by a continuous engagement interface between the interferencedevice and the bone screws.
 2. The bone plate system of claim 1, whereinthe screw slots are approximately the same length.
 3. The bone platesystem of claim 1, wherein the interference device is attached to thebase plate with a cover screw that extends through a cover screw hole inthe interference device and into a cover screw hole in the base plate.4. The bone plate system of claim 3, wherein the cover screw hole in thebase plate is offset from the bone screw slots in the base plate.
 5. Thebone plate system of claim 1, wherein the interference device isattached to the base plate at a location that is offset from the bonescrew slots in the base plate.
 6. The bone plate system of claim 1,wherein the base plate has a bottom surface that is slightly concavealong its length and along its width.
 7. The bone plate system of claim1, wherein: each bone screw head has a generally rounded top surfacewith a center region and a generally rounded bottom surface; each screwslot in the base plate has a generally rounded edge surface thatinterfaces with the bottom surface of a corresponding bone screw head;and the interference device includes a generally rounded edge surfacethat interfaces with the generally rounded top surface of acorresponding bone screw head while not substantially covering thecenter region of the top surface of the bone screw head.
 8. The boneplate system of claim 7, wherein, on each bone screw, the rounded topsurface extends to a maximum diameter and the rounded bottom surfaceextends to a maximum diameter that is greater than the maximum diameterof the rounded top surface.
 9. The bone plate system of claim 7, whereinan interface between the top surface of the bone screw head and the edgesurface of the interference device requires a minimum force ranging fromabout 15 to about 50 pounds to cause the bone screw to slide within thescrew slots of the base plate.
 10. The bone plate system of claim 7,wherein an interface between the top surface of the bone screw head andthe edge surface of the interference device requires a minimum forceranging from about 30 to about 40 pounds to cause the bone screw toslide within the screw slots of the base plate.
 11. The bone platesystem of claim 7, wherein the interference device is attached to thebase plate at a location that is offset from the bone screw slots in thebase plate.
 12. The bone plate system of claim 1, wherein the base plateis a generally rectangular plate having two elongated generally parallelsides, a top end and a bottom end, with the top end being generallyparallel to the bottom end and shorter than the sides, the base platecomprising a first pair of screw slots near the top end, the first paircomprising the two generally parallel elongated screw slots extendingthrough the base plate, and a second pair of screw slots near the bottomend, the second pair of screw slots comprising two generally parallelelongated screw slots extending through the base plate; and wherein thesystem comprises a first pair of bone screws comprising the two bonescrews capable of securing the base plate to a bone by insertion throughthe first pair of screw slots, and further comprises a second pair ofbone screws comprising two bone screws capable of securing the baseplate to a bone by insertion through the second pair of screw slots,each bone screw of the second pair having a screw head and a threadedportion extending therefrom.
 13. The bone plate system of claim 12,wherein the bone screw slots of the first pair are equal in length tothe bone screw slots of the second pair.
 14. The bone plate system ofclaim 12, wherein the base plate has: a center region of generallyuniform thickness, the center region encompassing a portion of the firstpair of screw slots and a portion of the second pair of screw slots; atop end adjacent the center region and partially encompassing a portionof the first pair of screw slots, the top end being generally tapered sothat it is thicker closer to the slots and thinner closer to the edge ofthe plate; and a bottom end adjacent the center region and partiallyencompassing a portion of the second pair of screw slots, the bottom endbeing generally tapered so that it is thicker closer to the slots andthinner closer to the edge of the plate.
 15. The bone plate system ofclaim 14, wherein the center region has a thickness ranging from about 1mm to about 4 mm.
 16. The bone plate system of claim 15, wherein atleast one of the top and bottom ends tapers to an edge thickness rangingfrom about 0.5 mm to about 2 mm.
 17. The bone plate system of claim 14,wherein the center region has a thickness ranging from about 2 mm toabout 3 mm.
 18. The bone plate system of claim 17, wherein the at leastone of the top and bottom ends tapers to an edge thickness ranging fromabout 0.5 mm to about 1.5 mm.
 19. The bone plate system of claim 14,wherein at least one of the top and bottom ends tapers at an angleranging from about 20° to about 40°.
 20. The bone plate system of claim14, wherein the center region has a thickness ranging from about 1 mm toabout 4 mm and the top and bottom ends each taper to an edge thicknessranging from about 0.5 mm to about 2 mm at an angle ranging from about20° to about 40°.
 21. The bone plate system of claim 14, wherein thecenter region has a thickness ranging from about 2 mm to about 3 mm andthe top and bottom ends each taper to an edge thickness ranging fromabout 0.5 mm to about 1.5 mm at an angle ranging from about 20° to about40°.15. The bone plate system of claim 12, wherein the center region hasa thickness ranging from about 2 mm to about 3 mm.
 22. The bone platesystem of claim 21, wherein each of the top and bottom ends includes aconcave region.
 23. The bone plate system of claim 14, wherein at leastone of the top and bottom ends includes a concave region.
 24. The boneplate system of claim 14, wherein the interference device attached tothe base plate retains the first pair of bone screws; and wherein thesystem further comprises a second interference device attached to thebase plate retaining the second pair of bone screws.
 25. The bone platesystem of claim 24, wherein the interference device has an edge regionpositioned nearest the top end of the base plate that is tapered in adirection from a center of the interference device to an edge of theinterference device positioned nearest the top end of the base plate,and wherein the second interference device has an edge region positionednearest the bottom end of the base plate that is tapered in a directionfrom the center of the interference device to an edge of theinterference device positioned nearest the bottom end of the base plate.26. The bone plate system of claim 24, wherein the second interferencedevice is attached to the base plate at a first attachment location thatis offset from the first screw slots in the base plate, and furtherwherein the interference device is attached to the base plate at asecond attachment location that is offset from the second screw slots inthe base plate.
 27. The bone plate system of claim 26, wherein the firstand second attachment locations are each offset in a direction towardthe center of the base plate.
 28. The bone plate system of claim 14,wherein the first pair of bone screw slots are generally aligned withthe second pair of bone screw slots and the pairs of slots positioned soas to create a generally symmetrical arrangement about the center of thebase plate.
 29. The bone plate system of claim 14, wherein: each bonescrew head has a generally rounded top surface with a center region anda generally rounded bottom surface; each screw slot in the base platehas a generally rounded edge surface that interfaces with the bottomsurface of a corresponding bone screw head; and each interference deviceincludes two generally rounded edge surfaces, each of which interfaceswith the generally rounded top surface of a corresponding bone screwhead while not substantially covering the center region of the topsurface of the bone screw head.
 30. The bone plate system of claim 29,wherein each bone screw can toggle within the screw slots in the baseplate, while the interference device still exerts sufficient force onthe bone screw to control the degree to which the bone screw slideswithin the screw slot.
 31. The bone plate system of claim 29, wherein aninterface between the top surface of the bone screw head and the edgesurface of the interference device requires a minimum force ranging fromabout 15 to about 50 pounds to cause the bone screw to slide within thescrew slots of the base plate.
 32. The bone plate system of claim 31,wherein the interference device is attached to the base plate at a firstattachment location that is offset from the first screw slots in thebase plate, and further wherein the interference device is attached tothe base plate at a second attachment location that is offset from thesecond screw slots in the base plate.
 33. The bone plate system of claim32, wherein the first and second attachment locations are each offset ina direction toward the center of the base plate.
 34. The bone platesystem of claim 29, wherein an interface between the top surface of thebone screw head and the edge surface of the interference device requiresa minimum force ranging from about 30 to about 40 pounds to cause thebone screw to slide within the screw slots of the base plate.
 35. Thebone plate system of claim 14, wherein the system is weight-sharing overa period of at least about three months.
 36. The bone plate system ofclaim 14, wherein the system is weight-sharing over a period of at leastabout six months.
 37. A bone plate system including: a base plate havingan aperture for location adjacent to a bone member; a bone screwextending through the aperture for engaging the bone member; means forpermitting movement of the bone screw relative to the base plate and forvarying a resistance against the movement as the bone screw movesrelative to the base plate.
 38. A bone plate system as set forth inclaim 37, wherein the means for permitting movement and for varyingresistance includes means for providing a variable force pressingagainst a portion of the bone screw as the bone screw moves relative tothe base plate.
 39. A bone plate system as set forth in claim 38,wherein the means for providing a variable force provides the variancein force as the bone screw moves along a direction relative to the baseplate.
 40. A bone plate system as set forth in claim 37, wherein theaperture is an elongate slot, and the means for permitting movement andfor varying resistance includes an interference device that provides avariable resistance to movement of the bone screw as the bone screwmoves along the elongate slot.
 41. A bone plate system as set forth inclaim 40, wherein the interference device includes a member secured tothe base plate proximate to one end of the elongate slot and extendingover a portion of the bone screw.
 42. A bone plate system including: abase plate having an elongate slot for location adjacent to a bonemember; a bone screw having a portion for extending into the bonemember, a portion extending through the the slot, and a portion engagedfor retention of the bone screw; and a member attached to the base plateat a point adjacent to one end of the slot, the member having a portionextending from the attachment point along the slot and engaged with theengaged portion of the bone screw; the engaged portion of the bone screwand an area at least partially bounded by the member being sized suchthat the engaged portion of the bone screw is within an interference fitbetween the base plate and the member is greatest proximate to theattachment point.